(Reuters) -- U.S. troops are hard at work rehabilitating this battle-scarred, rubble-strewn airfield as a logistics and support hub for Iraqi and international forces in the decisive battle against Islamic State for the city of Mosul 60 km to the north.

The whirl of activity and the return of American soldiers signify a new U.S. build-up in Iraq 14 years on from the invasion that set off a conflict which has undergone various permutations.

But senior officers insist this mission is limited and temporary. The stated goal is to annihilate Islamic State and to help the Iraqi army.

"They are a sovereign nation and they've allowed us to come and advise them. We want to get rid of the bad guys. We are all moving towards the same objective," said Lieutenant Colonel Elizabeth Curtis of the 82nd Airborne support battalion.

Nine months ago, Islamic State still held Qayyara West. The hardline militants had seized it from the Iraqi army in 2014 and destroyed the place, demolishing buildings and breaking up the runway with jackhammers.

A resurgent Iraqi army recaptured it last July and soldiers from the U.S. 101st Airborne Division were deployed here in October as the offensive to recapture Mosul, Islamic State's last stronghold in the country, got underway. The 82nd Airborne took over at Q-West, as the base is known, in December.

About 1,000 personnel, mostly Americans but including other members of the international coalition, are based at Q-West out of a total of about 1,700 in the area of the Mosul operation, base commander Lieutenant Colonel Sebastian Pastor said.

Designated in military parlance as an Intermediate Staging Base, it provides support and logistics for several Tactical Assembly Areas closer to the battlefront. U.S. advisors are out on the field but Q-West also has an offensive role --- a rocket battery is stationed here and regularly fires missiles at IS positions in western Mosul, and an air cavalry troop flies its helicopters in support of Iraqi forces on the ground.

POST APOCALYPSE

The place has a post-apocalypse look to it. Piles of rubble from destroyed buildings dot the landscape. Concrete blast walls surround the perimeter and snake through the inside. Containers and pallets of supplies are piled everywhere and fleets of armoured vehicles and bulldozers are parked in rows.

The soldiers sleep in small concrete bunkers under tenting. A toppled water tower has become a landmark. In recent days, heavy rains had turned the ground into a sea of mud.

It is a far cry from when it was a major U.S. base at the height of the occupation, with a golf-driving range and a swimming pool, and was dubbed Key West after the Florida holiday island.

"It's going to get bigger but it's not going to get nicer," said battalion planner Captain Anne Nagy, who is in charge of construction.

Nagy described the base as a giant Greyhound bus station.

"We have hundreds of people in transit and they need supplying. You have a lot of people who are basically dedicated to organising people, food, fuel, ammunition."

More troops will be arriving but how many and for how long depends on the battle for Mosul and its aftermath, Nagy said.

"We are going to hand it over to the Iraqi army as this operation closes out," she said.

Lieutenant Colonel Curtis also stressed that the build-up did not mean the United States was committing its forces to a long and costly new era of involvement.

She had served in Iraq during the 2003 invasion.

"It's a different fight now. We are here to support the Iraqi government. We are here on their permission and at their request. We are not the ones doing the fighting. We are advising them."

Asked how long they might stay, she said. "Our fight right now is in west Mosul. We can't speculate on the way ahead."

Handing over the airfield to Iraqi control is also a principle aim, said Air Force Lieutenant Colonel Carrie Coleman, head of the 370th Air Expeditionary Advisory Group detachment.

The runway has been repaired and can handle giant C-130 transporters. Two or three land per day, mostly at night for security reasons. There are air traffic controllers, weather specialists and cargo teams.

"Our mission here is to keep the logistics moving, bringing in cargo and people. Our goal is to get the Iraqis on their feet as soon as we can," Coleman said.